Timing is everything. Even how many calories someone burns while resting can depend on the time of day, new data show. These findings add to evidence that when people eat and sleep may be as important as what they eat for maintaining proper health.
People burn about 129 more calories when resting in the afternoon and evening than they do resting in the early morning. Morning is better for burning carbohydrates — sugars and starch. The body is more likely to burn fats in the evening.
Calories burned at rest fuel breathing, blood circulation and brain activity. They also help maintain body temperature. But there had been conflicting data on whether people at rest burn calories at a fairly constant rate. Some data suggested the rate might rise and fall in a daily — or circadian (Sur-KAY-dee-un) — rhythm. Metabolism is the rate at which the body burns calories to fuel activities.
The new study shows that a body’s resting metabolism is governed by circadian clocks.
Jeanne Duffy is a neuroscientist. She works at Brigham and Women’s Hospital in Boston, Mass. She and her colleagues followed seven people. They remained in windowless rooms for three weeks. That kept them from getting any clues to the time of day. Each night, the seven went to bed four hours later than the night before. The schedule change allowed the researchers to study the natural body rhythms of each subject without outside influences (such as sunlight and darkness).
Study recruits all showed clear rhythms for when they burned calories. But the timing of the highest and lowest rates could vary from person to person. For instance, calorie burning at rest peaked on average around 5 p.m. But some people peaked earlier, at around 2 p.m. Others peaked around 8 p.m. Calorie burning was lowest around 5 a.m. (but ranged from about 2 a.m. to 8 a.m., depending on the person).
That variability is normal for circadian rhythms, Duffy says. After all, some people are morning people. Others may be night owls. The timing of their daily rhythms reflects those differences.
“Regularity is really important,” Duffy says. Irregular schedules interrupt circadian rhythms. That in turn can throw off your metabolism, which can cause people to burn fewer calories. Studies have already shown that shift work (working at night) and chronic sleep loss can lead to weight gain and health problems. It doesn’t matter exactly what time people get up or eat, Duffy says. The most important thing is that they keep to a regular schedule. This includes the weekends — no matter how tempting it is to sleep in.
Duffy’s team shared its findings November 8 in Current Biology.